You can change your site main language via Settings -> General -> Site Language
Theme language files are located in a ../wp-content/themes/theme-folder/languages/ folder.
There are several ways to fill up translations:
- It’s easy to localize your site using Loco Translate plugin. Here you can see the tutorial – https://localise.biz/help/wordpress/translate-plugin/beginners.
- Besides this, you can use a PoEdit platform. You can add translation via editing your .po file and at the end, PoEdit generates .mo file for you. Then you can upload ready translation files to primordial folder (../wp-content/themes/theme-folder/languages/). Before adding translations for static strings it needs to rename language files correctly according to your site languages. You can find a list of language codes at WordPress in your Language. As an example, the language code for French is fr_FR, so you would save the translated files as fr_FR.po first, then fr_FR.mo.
In order to safe your translations during theme updates, we recommend to save them in ../wp-content/languages/themes/ folder.